GB797810A - Improvements in or relating to pulse controlled electrical circuit arrangements - Google Patents

Abstract

797,810. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 23, 1956 [Jan. 11, 1956], No. 973/56. Class 40 (3). [Also in Group XL (c)] In a pulse-controlled circuit arrangement, the control pulses are eliminated within whose duration impulsive noise occurs. Fig. 2 shows the application to a television video signal clamping circuit in which the signal to be clamped which has positive-going picture component and negative synchronizing signals is applied at 2 to a conventional double diode clamp 4 and the clamping pulses are applied at 7 to an " immunization circuit " 6 which removes a clamping pulse when noise occurs simultaneously. The output from circuit 6 is amplified at 8 and passed to a phase-splitter 5 the output from which controls clamp 4. An " immunization circuit " for protecting against positive-going noise pulses only is shown in Fig. 3. The video signal applied at input 9 is D.C. restored by the circuit comprising diode 13 and applied to tube 11 which conducts only when a clamping pulse cuts-off tube 12 so that pulses appear at its anode whose amplitude is proportional to the difference between the video signal back-porch level and the constant D.C. restoration level. These pulses are passed by clipper diode 14 so that when a positive noise pulse is present in the video input during the back-porch period the pulses at the anode of tube 11 will have an amplitude below the clipping level of diode 14 and no pulse appears at point 10 for that period. Fig. 4 shows a circuit for protecting against noise of any polarity in which the input video signal is applied to a subsidiary clamp circuit 16 the output of which is applied via phase-splitter 18 which as shown in Fig. 5 comprises a tube 5A the outputs across the anode and cathode circuits of which are applied to a load 5D via rectifiers 5B, 5C. The output from stage 18, Fig. 4, is applied to control a gate 19 so that clamping pulses applied at 7 pass to output 10 only when there is no control pulse from stage 18. The clamping stage 16, may, as described in Specification 797,809, and shown in Fig. 6, comprise a cathodefollower 6A to which the video input signal is applied the output from which is clamped by double-diode 6B and applied via cathodefollower 6D to be subtractively combined with the unclamped video signal in circuit 6E, 6F to produce an output pulse at 6H dependent in time and amplitude upon the impulsive noise in the video input signal. When low frequency interference such as mains hum is present on the video signal the circuit of Fig. 2 may be modified by feeding the " immunization circuit " 6 from the clamped output of clamp 4 so that the L.F. interference is substantially removed by the clamp circuit. A further reduction of the L.F. interference may be produced by the circuit shown in Fig. 7 in which the output of clamp 16 is as described in Specification 797,809, a stepped L.F. signal on which is superimposed pulses corresponding to impulsive noise. This signal is differentiated in circuit 20 and fed to phase-splitter 18, clipper 21, the output from which triggers a rectangular pulse generator 22 to operate gate 19.